In skeletal muscle, the mechanisms coupling the action potential across the sarcolemma and transverse tubular (T-) membranes to initiation of calcium release from the sarcoplasmic reticulum (SR) remain unknown. This represents the major gap in our knowledge of the regulation of muscle contraction. The morphological pathway for this coupling is thought to be the anatomically well-defined T:SR junctions (e.g. diads and triads), but these junctions represent only a small part of the total muscle cell membrane area and generally are buried deep within a structurally and electrically complicated T-system. The chief aim of this proposal is to study these junctions in the most direct way technically possible. An extracellular patch-clamp electrode (lMu dia,) will be employed to make electrophysiological measurements from small regions of muscle cell membranes naturally enriched in these junctions. This technique will be utilized with certain vertebrate and invertebrate preparations in which T:SR junctions are in close physical and electrical proximity to the cell surface. This approach will provide the most direct electrical recordings to date from these poorly understood junctions. Success in electrical measurements from a single T-tubule from scorpion muscle or from a single surface membrane:SR junction in frog slow muscle will give a direct indication of what electrical activity underlying T:SR Coupling looks like and localize this activity to junctional membrane. These experiments will also enable characterization of the activity in order to determine whether the currents are capacitive, ionic, or both and to determine whether extracellular calcium plays an important role in their generation. These answers are basic to discovering the mechanisms involved in T:SR Coupling. They are also fundamental to understanding the role this process might play in both cause and treatment of muscle disorders.